Acid-peroxide dissolution of metals in the presence of titanium



United States Patent O 3,483,050 ACID-PEROXIDE DISSOLUTION OF METALS IN THE PRESENCE OF TITANIUM Russell S. Banush, Syracuse, N.Y., assignor t Allied Chemical Corporation, New York, N.Y., a corporation of New York No Drawing. Filed Mar. 17, 1966, Ser. No. 535,012 Int. Cl. C23f 11/04 US. Cl. 156-18 13 Claims ABSTRACT OF THE DISCLOSURE The inhibition of the corrosion of titanium and titanium alloy equipment used in the dissolution of metals with an acidfied hydrogen peroxide etching composition is achieved by incorporating in the acidified hydrogen peroxide solutions a minor amount sufficient to reduce the dissolution of said titanium or titanium alloy to less than 8 mils per year of at least two metals selected from bismuth, antimony and arsenic.

Aqueous solutions of hydrogen peroxide and mineral acid are especially useful in dissolving metals such as in etching of copper in the manufacture of printed circuit boards. Such aqueous solutions contain generally less than a 50% total concentration of peroxide and acid, and typically contain 212% hydrogen peroxide and 223,% sulfuric acid. As etchants these solutions are of particular interest because of ability to dissolve metals at high rates and at a marked reduction in cost over other materials. The ability of such solutions to dissolve at high rates is indicative of strength of the solutions but raises the possibility that the solution might undesirably attack other materials with which it comes in contact during use. One problem of this type has been found in etching of printed circuit boards in situations where the etching is carried out in equipment constructed at least in part of titanium which is subject to corrosive at tack by the acid-peroxide etchant solutions.

An object of the present invention is to reduce the rate of dissolution or corrosion of titanium by aqueous solutions of acid and peroxide.

Another object is to provide etchants and methods for dissolving copper in the presence of titanium without undesirable corrosive attack on the titanium.

A further object is to provide chemical additives effective in reducing the dissolution by aqueous acid-peroxide etchants of titanium without substantial reduction of the effectiveness of the etchant in dissolving metals.

Other objects and advantages will be evident from the following description of the invention.

In accordance with the invention it has been found that the dissolution of titanium metal by aqueous acidperoxide etchant solutions is effectively reduced by incorporating in said solutions a combination of ions of at least two metals selected from the group consisting of bismuth, arsenic and antimony. Only minor amounts of the metal ions are required in combination to reduce the dissolution of titanium to a tolerable rate of less than about 8 mils of surface per year as measured at 60 C. in an aqueous solution containing 6% hydrogen peroxide and 13% sulfuric acid. Moreover, the presence of the metal additives in the acid-peroxide solution acts not only selectively on the titanium in that the effectiveness of the etchant is not repressed but also has been found to extend the capacity of the etchant over solutions not containing the additives.

The metal ions are preferably introduced into the aqueous etchant solutions in the form of a fully or partially soluble oxide or salt which ionizes to yield the ions in solution. Examples of suitable compounds include bismuth nitrate, sodium bismuthate, bismuth 1- lactate, bismuth sulfate, bismuth chloride, antimony trioxide, antimony pentoxide, and arsenic pentoxide. The preferred forms of each of the additive compounds are bismuth nitrate, antimony trioxide and arsenic pentoxide and the concentrations given herein for the respective metal additives are expressed in terms of the dissolved amounts of these compounds. Compounds which ionize with difficulty or to a very limited extent in ordinary water may be effectively employed in the invention because of their increased dissociation in the acidified solutions, especially at the temperatures normally employed in etching, e.g. 4065 C. Additives such as antimony trioxide and arsenic pentoxide which dissolve slowly may be more readily added to the etchant by first dissolving in a hot solution of the acid used to make up the etchant. The amount of the additives employed may vary over a fairly wide range. Generally, at least about 200 parts per million total additives is required with a minimum of at least 25 parts per million of each additive required to produce an effective combination. The upper limit of the total amount of additives is mostly a matter of economies with amounts in excess of about 6,000 parts yielding no substantial additional benefit and only resulting in increased cost. The optimum amounts of the individual additives depend largely on the acid concentration of the etchant which is the primary factor controlling the degree of attack on the titanium. Generally, the higher acid concentrations cause the greater attack on the titanium and require the higher additive concentrations. The more preferred additive combination is bismuth and antimony with concentrations generally ranging between about 400-5,500 parts per million of bismuth expressed as bismuth nitrate and 2,000 parts per million of antimony expressed as antimony trioxide. The preferred dissolved concentration of bismuth and antimony is 10003500 parts per million bismuth nitrate and 500 parts per million antimony trioxide. Another preferred combination is antimony trioxide and arsenic pentoxide each in a dissolved concentration ranging between 1002000 parts per million, more preferably 150-500 parts per million. Each of the two above preferred combinations has been found to reduce the corrosion or dissolution of titanium in acid-peroxide to at least a negligible rate.

In the acid-peroxide solutions of the invention the hydrogen peroxide concentration may vary over a fairly wide range. Etching of copper metal is desirably carried out in acidified solutions having a hydrogen peroxide concentration between about 212% The best results are obtained in solutions having a peroxide concentration between about 210%. During the etching process hydrogen peroxide is consumed as more and more amounts of metal are treated. In order to be practical it is necessary that a single etchant dissolve a substantial amount of copper metal before the solution becomes exhausted to the extent that a particular workpiece cannot be etched within a reasonable time, e.g. 1-2 hours. The hydrogen peroxide solutions must therefore have an initial hydrogen peroxide concentration of at least about 4% in order to dissolve suflicient metal to be practical from an economic standpoint. Desirably, the etchant solution has initially a hydrogen peroxide concentration within the range of about 510% by weight. The hydrogen peroxide-acid solutions are capable of operating effectively at good etch rates after partial exhaustion and even at high dissolved copper concentrations equivalent to at least ounces of copper per gallon and even substantially higher.

The acid concentration may also vary considerably. In copper etching it is desirable that the etchant solution have a hydrogen ion concentration from about 0.45 to about 5.5 grams per liter, preferably between about 0.65 4.5 grams per liter. Below a hydrogen ion concentration of about 0.45 gram per liter the etch rate is slow and peroxide decomposition high. The desired upper limit of the hydrogen ion concentration may depend on several factors including the particular acid employed. A hydrogen ion concentration above about 5.5 grams per liter is generally less economical and tends to slow down rather than increase the etch rate. The high acid concentrations are also less desirable as tending to increase attack on the titanium and requiring the larger amounts of the metal additive to control such attack at a tolerable level. Inorganic acids and even the stronger organic acids such a9 acetic acid may be used to supply the hydrogen ion concentration in the etchant solution. Examples of the acids which are the more suitable include sulfuric acid, nitric acid, and fluoboric acid, preferably, sulfuric acid. The acid preferably employed in peroxide etching of copper is sulfuric acid. The amount of sulfuric acid in the hydrogen peroxide etchant is between about 223% by weight, preferably between about 3-20% by weight. Sulfuric acid concentrations above about 23% are less desirable due to a tendency to result in less uniform etching.

In the etchant solutions the ratio of hydrogen peroxide to acid is less important than the concentration of the acid. As the chemical reaction or mechanism by which copper is etched consumes one mol of peroxide and 2 mols of acid hydrogen, a mol ratio of 1 to 2 is indicated, i.e. a H O /l-H- ratio of l to 2. Peroxide to hydrogen ion mol ratios less than 1 to 2 are therefore generally unnecessary and may tend to slow the etch rate, particularly at the higher reagent concentrations. In practice, the amount of hydrogen peroxide actually consumed seldom will exceed about 75% so that the inclusion of just slightly more than about 1.5 mols of hydrogen ion per mol of peroxide will be adequate to supply sufficient acid for complete utilization of the particular etchant solution. As some peroxide is also not utilized because of decomposition the etchants made up to include sufficient acid for complete utilization without addition of more acid preferably have a hydrogen peroxide to hydrogen ion mol ratio of not less than about 1.0 to 1.6, and desirably in the range of about 1.0216 to 1.0:1.0. When acid is to be later added and the etchant solution contains initially a low or intermediate acid concentration the mol ratio of peroxide to acid hydrogen may, of course, be initially somewhat greater, preferably between about 10:02 to 1.0: 1.0. As hydrogen peroxide is consumed and more acid added the mol ratio of peroxide to acid will be reduced and eventually become similar to the mol ratios preferably employed in the solutions made up to contain the complete acid requirement. Again, because peroxide utilization seldom exceeds 75%, it is desirable from a practical viewpoint not to add an amount of acid sufiicient to reduce the mol ratio of peroxide to acid hydrogen below about 1.0 to 1.6.

Temperature of the acidified-hydrogen peroxide solution is another important factor in etching. As a practical matter metals such as copper are not etched at room tem peratures or below. The nature of the attack of the acid hydrogen peroxide solution on copper at such temperatures is more of a polishing, oxidizing or brightening effect. In order to efficiently etch copper the hydrogen peroxide solution must have a temperature of at least about 40 C. at time of contact with the metal. Solution temperature has a strong effect on etch rates and increasing the temperature to a preferred range between about 50- 62 C. will substantially increase the rate of etching. At hydrogen peroxide solution temperatures above about 65 C. little further increase in etch rate is realized and such temperatures have been found less desirable as resulting in an impractically high rate of peroxide decomposition. If desired, etching may be commenced at the lower temperatures, for example, between about 40 C. to 55 C., and temperature of the solution then gradually increased up to a higher temperature of approximately 55-62 C. as the solution is further exhausted. Increasing the temperature of the etchant solution is aided by the etching reaction itself which is moderately exothermic. Increasing the temperature of the etchant may be used to advantage to regulate etch rates at a more or less constant value when a number of pieces are to be etched in the same solution such as, for example, when employing automatic systems used in the manufacture of printed circuits.

The acid-peroxide etchant solutions desirably also contain one or more additives adapted to promote the dissolution of the metal to be treated by the solution. Phenacetin, phenylurea, diphenylurea, benzoic acid, sulfathiazole and silver ion have been found to be highly effective in improving the etch rate and capacity of acid-peroxide solutions. Salts yielding these additives in the acidperoxide solution may also be employed. For example, the sodium salt of sulfathiazole and benzoic acid may be added to thesolution. Silver nitrate and other soluble salts such as silver sulfate will furnish silver ions. In etching or dissolution of copper metal special consideration is generally given to provide a hydrogen peroxide etchant solution in which little or no chloride and bromide ion is present. For example, deionized water may be used to make-up an etchant containing less than 2 parts per million of chloride and bromide ions. Or, if desired, ordinary water may be employed in make-up of the etchant solution it accompanied by addition of suitable material capable of removing free chloride and bromide ions. A small amount of a water-soluble silver salt, preferably silver nitrate, is added to effect the removal of chloride and bromide ion. The precipitated silver halide matter is allowed to remain in the acid-peroxide-phenacetin solution and does not interfere with the etching process. The addition of excess soluble silver salt will furnish free silver ions in the etchant and have a highly beneficial and catalytic effect upon etch rate and capacity. When phenacetin is used alone for copper etching it is important that the aqueous hydrogen peroxide etchant solution contain less than 2 parts per million total free chloride and bromide ions, preferably less than 1 part per million. The addition of sulfathiazole has the advantage of permitting use of ordinary tap water in preparation of the etchant Without special treatment as required when phenacetin is used alone. When employing sulfathiazole either in combination or alone in etchants prepared from ordinary tap Water, between about to 250 parts per million are usuallp required to negate the adverse effects of the free chloride and bromide ions imthe water. The capacity of sulfathiazole in overcoming the adverse effect of chloride and bromide ion is apparently not unlimited. Solutions containing the higher concentrations of these ions, say above about 20-30 parts per million, will require additional treatment to negate this effect of such ions, e.g. deionization or addition of a soluble silver salt. Combinations of the various additives are especially effective in improving the etch rate and capacity of the acid-peroxide solution. Examples of preferred combinations are phenacetin with silver nitrate, phenacetin with sulfathiazole, and phenylurea and/or diphenylurea with sulfathiazole and/ or silver nitrate.

In preparation of the acidified peroxide etchant only a small amount of the catalytic additives are required to have the desired catalytic effect. As little as about 50 parts per million of phenacetin or sulfathiazole may be used in providing an etchant of improved capacity. Somewhat lesser amounts of free silver ion may be effective, particularly in immersion etching procedures where as little as about parts per million would be effective. Increasing the amount of additive will further increase etch rate. About 200-1,000 parts per million of phenacetin or sulfathiazole is preferred. About 50-500 parts per million of free silver ion generally represents a preferred amount. The upper limit of the amount of additive is not critical and mostly a matter of economics. Generally, an amount of additive in excess of about 5,000 parts per million offers no added advantage and is undesirable from a process and economic standpoint. When using mixtures of phenacetin, phenylurea, benzoic acid, sulfathiazole or a material furnishing silver ions, at least about 25 parts per million of each are employed. Preferably the mixtures total about 200-1,500 parts per million of additive with between about 100-1,000 parts per million of each being employed.

The present invention is applicable to reducing the dissolution rate of relativel ure titanium and alloys of titanium. Effectiveness in reducing the attack on alloys of titanium varies depending on the alloy composition. The additive combinations have been found highly effective with certain commercially known alloys such as 92.5% titanium, 5% aluminum, and 2.5% tin; 73% titanium, 3% aluminum, 11% chromium, and 13% vanadium; 92% titanium and 8% manganese; and 99.85% titanium and 0.15% palladium. The combinations are less effective with other alloys such as 92% titanium, 4% aluminum, 3% molybdenum, and 1% vanadium; 90% titanium, 8% aluminum, 1% molybdenum, and 1% vanadium; 90% titanium, 6% aluminum, and 4% vanadium; and 89% titanium, 7% aluminum, and 4% molybdenum. The effectiveness of the particular additive combination at least with the titanium alloys has been found to vary with the particular etching procedure. Thus, the additive combinations are generally more effective in spray etching than in ordinary bath or immersion etching procedures.

The present invention is eminently suited for etching of copper in applications such as manufacture of printed circuit boards where titanium equipment is commonly employed. In addition to etching of copper the invention may be applied generally in other conventional chemical dissolving operations such as chemical milling, graining and bright dipping or polishing where titanium equipment might be used. In such applications the temperature of the acid-peroxide solution may be varied, if desired, outside of the range prescribed for the etching of copper. For example, bright dipping operations may be carried out effectively at room temperature or slightly above. The acid-peroxide solutions of the invention may also be employed in the dissolution of copper alloys and of other metals such as iron, nickel, cadmium, zinc, germanium, lead, steel, aluminum and alloys containing a major portion of such metals. Aluminum metal is more effectively dissolved when the acid employed is nitric acid or fluoboric acid, particularly fluoboric acid.

The following examples in which parts and percentages are by weight unless otherwise noted demonstrate the practice and advantages of the present invention.

EXAMPLES 1-10 In Examples 1-10 panels of CF. titanium measuring 1 inch x 6 inches were cleaned by a 10 second dip in dilute (10% by volume) HNO -HF solution and then rinsed with water and acetone. After drying and weighing the specimens were partially immersed to a depth of 3 inches in 200 ml. of etchant solution contained in a beaker so that 6 square inches of the specimen was exposed to the solution. The etchant was an aqueous solution containing 6% hydrogen peroxide, 13% sulfuric acid, 200 parts per million silver nitrate, 96 p.p.m. phenacetin and 180 parts per million adipic acid. Additives were incorporated to inhibit the dissolution of the titanium as indicated below in Table I. The beaker was placed in a water bath maintained at a temperature of 60 C. Evaporation losses from the test solution were made up by addition of water. Each specimen was immersed in the solution for 24 hours and dissolution of the titanium was determined by weight difference between the specimen before and after the test. The weight loss was converted to a dissolution rate expressed in mils of titanium surface per year.

TABLE I Percent reduction Titanium in Additive in total amount of dissolution titanium 2,000 parts per million rate mls. dissolution Example N0. except as noted per year rate 1 None 813203. 33 59 SbzO5 41 49 AS2O5 24 70 NaBiO3. 14. 2 82 6 Bl(N03)3-5H20 20 75 7 .J'1,50() p.p.m. i(N 3)3.5H2O. 1 0

I500 p.p.m. $13203 J 8 ,500 p.p.m. Bi(NOa)a.5HzO 5. 6 93 500 p.p.m. AS205" 9 {2,000 p.p.m. $10 0 O 100 2,000 p.p.m. AS205. 10 ..{2,000 p.p.m. NaBiOz. 0 100 2,000 p.p.m. SbzOa As shown by Table I the dissolution rate of titanium in the presence of acid and hydrogen peroxide at 60 C. is a high 80 mils of surface per year. Examples 2-6 show that the additives employed in the invention are individually effective in reducing the titanium corrosion but fall well short of lowering the rate to a practical level of only 8 mils per year. Examples 7-10 demonstrate the present invention and show unexpected reduction of the corrosion rate in acid-peroxide at elevated temperatures when the additives employed in Examples 2-6 are combined. Examples 7 and 8 show the combination of the additives in the same total amounts as employed in Examples 2-6. Example 7 shows that the combination of bismuth and antimony is effective in essentially arresting the dissolution of titanium under the severe conditions of the test bath. Example 8 shows that the combination of bismuth and arsenic at a total of 2000 p.p.m. is effective in reducing the titanium dissolution rate to a low 5.6 mils per year. Example 9 shows that the combination of antimony and arsenic each in an amount of 2000 p.p.m. (expressed as the oxides) is also effective in essenitally eliminating the attack of the acid-peroxide solution on the titanium. Example 10 shows the same desirable results employing a combination of bismuth and antimony with the bismuth being incorporated in the solution in the form of sodium bismuthate.

EXAMPLES 11-15 The following Examples 11-15 demonstrate the invention during the actual etching of copper by spray etching procedures in the presence of titanium and various titanium alloys. All aqueous sulfuric acid-hydrogen peroxide etchant solutions contained additives to accelerate etching and also had incorporated therein the combination of 1500 p.p.m. bismuth nitrate and 200 p.p.m. antimony trioxide to reduce dissolution of the titanium, as employed in Examples 1-10. The various specimens of titanium and titanium alloys in these examples were panels measuring 2 inches by 13.5 inches. All titanium specimens except that of Example 13 had a dissolution rate of greater than 50 mils per year in the acid-peroxide etchant without the bismuth and antimony additives as determined by static immersion tests similar to those of Examples 1-10. Each specimen was placed in the vicinity of the spray and etching zones so that it was'fully subject to wetting by the etchant solution. Dissolution rate of the titanium specimens was determined by Weight difference between the specimen before and after the test which was of 12 hour duration. The Weight loss was converted to a titanium dissolution rate expressed in mils of titanium per year. The copper etched during the test 7 was in the form of copper clad laminates obtained under the trademark Textolite (No. 11571) from the General Electric Company. In the following examples the copper clad laminates were cut into board specimens having dimensions of 9 x 9 x A inch. Each of these specimens per million of antimony trioxide and 150500 parts per million of arsenic pentoxide.

4. The method according to claim 1 in which the solution contains 212% by weight hydrogen peroxide and 223% by weight sulfuric acid.

was then spray etched using a Model 600 spray etcher 5. The method of claim 1 wherein the total concenmanufactured by the Chemcut Division of Centre Cirtration of additive metal is between 200 and 6,000 parts cuits Company (U.S.A.). The reservoir of the spray per million. etcher was charged with about 3 gallons of etchant solu- 6. The method of claim 1 wherein the acidified hydrotion and the spray etcher set to apply about 5 gallons per gen peroxide solution has incorporated therein between minute to each specimen. Etch time was determined with 400 and 2,000 parts per million of antimony trioxide. a stopwatch and etch rate calculated after weighing each 7. The method of claim 1 wherein the acidified hydrospecimen before and after treatment. Capacity of the gen peroxide solution has incorporated therein between etchant was determined on the basis of the weight of 100 and 2,000 parts per million antimony trioxide and copper in ounces dissolved or etched by the solutions dur- 100 to 2,000 parts per million of arsenic pentoxide. ing a period of 8 minutes. Examples 11-15 and the results 8. A composition for selective dissolution of a metal thereof are summarized below in Table II. selected from copper and copper alloys in the presence TABLE II Titanium Copper Bath Capacity, dissolution rate ounces/gallon mils year, etchant with Etchant Etchant with 200 ppm. without 200 ppm. SD; plus bismuth- $1120 plus Composition of titanium 1,500 ppm. antimony 1,500 p.p.m. test specimen BKNOQMHZO additive Bi(NOi)3.5H O

Example No.:

11--. (LP. Ti 0.0 8.9 9. 4 12 02.5 0 Ti, 5% A1, 2.5% Sn 5. 0 8.7 9. 0 13. 73% Ti, 3% Al, 11% Cr, 13% v.--. 0. 0 8.7 0. 3 14 02% Ti, 3% Mn 0.0 0.1 15 99.85% Ti, 0.15% Pd 0.0 0. 4

Table II shows that the bismuth-antimony additive of titanium and titanium alloys comprising an aqueous combination is effective in reducing the attack on all the titanium and titanium alloy specimens of Examples 11- 15. In all examples except Example 12 the additive combination substantially arrests the dissolution of the titanium materials by the acid-peroxide etchant. While the bismuth-antimony combination is effective in reducing the dissolution of the titanium the results of Examples 11-15 show that the combination is selective and has no advese effect on the etching or dissolution of the copper by the acid-peroxide. Surprisingly, available data in Example 11-13 show that the bismuth-antimony combination results in an improvement of etchant capacity of between about 37%.

Although certain preferred embodiments of the invention have been'disclosed for purpose of illustration, it will be evident that various changes and modifications may be made therein without departing from the scope and spirit of the invention.

1 claim:

1. A method for the dissolution of a metal selected from copper and copper alloys in the presence of titanium and titanium alloys which comprises contacting said metal with an acidified hydrogen peroxide solution containing 2 to 12% by weight hydrogen peroxide and 2% to 23% by weight sulfuric acid at a temperature of about 40 to 65 C., said solution having incorporated therein an amount sufiicient to reduce the dissolution of said titanium to less than 8 mils per year, of at least two metals selected from the group consisting of bismuth, antimony and arsenic.

2. The method according to claim 1 in which the solution has incorporated therein between 1000-3500 parts per million bismuth nitrate and between 150-500 parts per million antimony trioxide.

3. The method according to claim 1 in which the solution has incorporated therein between 150-500 parts solution of 4 to 12% by weight hydrogen, 2 to 23% by weight sulfuric acid and having incorporated therein an amount sufiicient to reduce the dissolution of said titanium to less than 8 mils per year of at least two metals selected from the group consisting of bismuth, antimony and arsenic.

9. The composition of claim 8 in which there is incorporated between 1000-3500 parts per million 01 bismuth nitrate and between 150-500 parts per million of antimony trioxide.

10. The composition of claim 8 in which there is incorporated between 150-500 parts per million of antimony trioxide and 150500 parts per million of arsenic pentoxide.

11. The composition of claim 8 wherein the total concentration of additive metal is between 200 and 6,00 parts per million.

12. The composition of claim 8 wherein the acidified hydrogen peroxide solution has incorporated therein between 400 and 5,500 parts per million in bismuth nitrate and between and 2,000 parts per million of antimony trioxide.

13. The composition of claim 8 wherein the acidified hydrogen peroxide solution has incorporated therein between 100 and 2,000 parts per million antimony trioxide and 100 to 2,000 parts per million of arsenic pentoxide.

References Cited UNITED STATES PATENTS JACOB H. STEINBERG, Primary Examiner Us. 01. X.R. 1563; 252-79.;

UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION Patent N' 3lu83lo5o Dated December 9 1969 Inventor(s) Russell 3 Banush It is certified that error appears in the above-identifiedpatent and that said Letters Patent are hereby corrected as shown below:

Column 4, line 56 "usuallp" should be --usually--.

Column 7, line 39 "advese" should be --adverse--.

Column 7, line 41 "Example" should be --Examples--.

Column 8, line 86 "6,00" should be --6,000--.

SIGNED AND SEALED Anon:

EdwardH-FletcherJr. I E u m. Awning Officer Gomissioner of Patents 

